The goal of this research is to develop models for predicting the toxicity and mobilization of individual metals[unreadable] and metal mixtures in sediments. These predictions are critical in evaluating the risk associated with[unreadable] contaminated sediments at Superfund sites. In past Superfund basic research projects, we have developed[unreadable] methods of assessing the toxicity of individual metals - Cd, Cu, Ni, Pb and Zn - for sediments with excess[unreadable] acid volatile sulfides (AVS). We propose to develop a model that predicts the toxicity of metals in sediments[unreadable] with little or no AVS. This requires a prediction of the partitioning of sediment metals to the other important[unreadable] sediment phases - sediment organic carbon if it is in sufficient supply, and then the other phases (e.g. iron[unreadable] oxyhydroxides) that are important in the aerobic layer of sediments. We also propose to develop the next[unreadable] generation of models for the prediction of mobilization of metals from sediments that explicitly include the[unreadable] mechanisms of mobilization. We intend to model the mechanisms of metal sulfide oxidation directly and to[unreadable] relate them to the cycles of manganese and iron in sediments. We have included such a mechanism[unreadable] nvolving the Fe(ll)-facilitated oxidation of arsenic in our present project. We propose to extend this work to[unreadable] the oxidation of metal sulfides.[unreadable] Our specific aims are:[unreadable] 1. For cationic metals, to extend the sediment Biotic Ligand Model (BLM) so that it can predict the toxicity of[unreadable] metals and metal mixtures by considering competitive interactions of metals and major ions to the organic[unreadable] carbon, (hydrous) oxide surfaces, clays and to uptake sites on benthic organisms.[unreadable] 2. For metals that form insoluble metal sulfides in sediments (Cd, Cu, Ni, Pb, Zn), to determine the rates of[unreadable] metal sulfide oxidation and their dependence on solution parameters for inclusion in metal mobilization[unreadable] models.[unreadable] 3. For arsenic, to investigate the effects of inorganic and organic ligands present in sediments that affect[unreadable] the rate of Fe(ll)-catalyzed oxidation of arsenite and ultimately the rates of arsenic mobilization from[unreadable] sediments.[unreadable] 4. For chromium, to investigate the effect of natural organic matter on the rates of chromium(lll) oxidation[unreadable] by manganese oxide in sediments, and the subsequent release of chromium(VI) to the pore water and the[unreadable] overlying water.[unreadable] 5. For the metals discussed above, to construct an integrated model that combines the metal mobilization[unreadable] and toxicity mechanisms, together with a model of the seasonal cycling of redox-sensitive species including[unreadable] oxygen, organic matter, iron, manganese and sulfur in sediments.